1. Field of the Invention
The present invention relates to a memory controller for controlling a synchronous memory and an image forming device provided with the memory controller.
2. Description of Related Art
Recent electric devices operate with the aid of a CPU of a personal computer (hereinafter referred to as “PC”) or a CPU incorporated in a printer or the like. The CPU processes and controls a variety of data. Operating frequencies of the system clock (reference clock) in the CPUs are constantly increasing to increase the processing speed. However, higher clock frequencies result in generation of greater electromagnetic interference in the device. Electromagnetic interference has a negative effect on other operations in the device and on other electronic devices.
In electronic devices, electronic components and wirings connecting the electronic components generate electromagnetic interference. The electromagnetic interference may affect other electronic devices. For example, an electronic device provided with a CPU has a memory control system. The memory control system has a memory control circuit. The memory control circuit controls a synchronous memory based on instructions issued from the CPU. In the memory control system, the memory control circuit and the synchronous memory are located some distance apart from each other on a single printed circuit board and are interconnected with each other by the wiring pattern.
In the memory control system, the electromagnetic interference is generated from: control signal wires for transferring control signals from the memory control circuit to the synchronous memory in order to control the synchronous memory operation; address signal wires for transferring from the memory control circuit to the synchronous memory address signals which are outputted from the memory control circuit simultaneously with the control signals; data transmission wires; and clock signal transmission wires. Clock signals are almost always supplied to the memory control circuit, the synchronous memory, the CPU, and other components in the electronic device. For this reason, the clock signal transmission wires are a main source of electromagnetic interference.
Recently, modulated clocks are used as operating clocks to suppress electromagnetic interference. Modulated clocks are generated by Spread Spectrum Clock (SSC) technology. The SSC technology modulates the reference clock to spread and fall within a narrow band such that the modulated clock periodically changes its clock frequency by a few percent. The change in clock frequency suppresses the peak level of electromagnetic interference, as disclosed in Japanese patent application publication No. 2000-280575.
An SDRAM (Synchronous Dynamic RAM) is often used as a main memory in computers because the SDRAM is cheap and is capable of storing a large amount of data. The SDRAM receives several types of commands from the memory control circuit. More specifically, as each type of command, the SDRAM receives one or more kinds of control signals constructing the subject command and an address signal adr corresponding to the subject command. The SDRAM receives the control signals and the address signal adr in synchronization with the clock signal clk. For example, as shown in FIG. 1, the SDRAM receives a chip select signal cs# (control signal) and the address signal adr in synchronization with the clock signal clk. The symbol “#” appended to the signal name “cs” indicates that the subject signal is a negative logic (active row).
As shown in FIG. 1, the control signal cs# is in the active state only during one period of the clock signal clk, that is, one clock cycle's worth of time. The address signal adr is brought into a valid address signal corresponding to the command only when the control signal cs# is in the active state.
In this way, the memory control circuit outputs the control signal and address signal adr as a command to the SDRAM only during one clock cycle. The control of SDRAM as described above is described in Japanese patent application publication No. 2003-101806.